The present invention relates to amplifiers used at the end of a signal path for providing an amplified signal to an output device and, more particularly, to such amplifiers which are to operate with a low quiescent current and also be capable of operating with a low supply voltage.
There are many situations in which electronic circuits must operate with only a low supply voltage being available thereto. In battery operated equipment, particularly, not only is the supply voltage often low, but also minimizing power dissipation is very important to lengthen the life of the battery or batteries used. Yet some of these situations require an output amplifier circuit which can deliver a considerable amount of power to the load device, perhaps aided by some capacitive storage provided across the power supply.
In these circumstances, the output amplifier type is often chosen to be such that the output transistors therein carry little or no current in the absence of an input signal to the output amplifier stage. This minimizes power dissipation during such conditions to thereby reduce the drain on the power supply and so leads to improved amplifier efficiency in converting power from such a supply to signal power.
One type of equipment where this has become increasingly important is in hearing aids, these having begun as a chest-worn package of electronics with a lead to an output device worn in the ear. The next major change was to a behind-the-ear electronics package, and then to an in-the-ear package and, finally, to an in-the-canal package. The reduction in size of the electronics package, including the battery power supply, to make such a transition in the locations on their person at which users of such hearing aids carry that equipment is quite remarkable. Microphones, speakers or receivers, batteries and electronic circuitry have all undergone varied but quite significant size reductions to make this possible.
However, further reductions in the physical size of these various portions of a hearing aid are desirable to increase benefits to the users thereof. A major difficulty in reducing the geometrical volume of the associated electronic circuitry has been the number of external capacitors needed in connection with output amplifier, the remaining circuitry of which is provided in a monolithic integrated circuit chip. Although such capacitors have also been reduced in size, they together still present major difficulties in miniaturizing hearing aid packages sufficiently to be insertable into smaller ear canals. Thus, there is desired an output amplifier circuit requiring little quiescent current which is capable of being operated at a low supply voltage, and which, as formed in a monolithic integrated circuit chip, reduces the number of external capacitors required to be used therewith.